This invention relates to an improved method for making cyanuric acid from urea.
It is known that cyanuric acid can be produced by the pyrolysis of urea. This reaction may be expressed by the equation: ##STR1## The resulting product, cyanuric acid, which has the empherical formula, C.sub.3 H.sub.3 O.sub.3 N.sub.3, is generally expressed structurally either as: ##STR2##
Pyrolysis can be carried out at temperatures above about 180.degree. C. either in a dry state that is, in the absence of a solvent, as is described in U.S. Pat. No. 2,943,088 issued to R. H. Westfall on June 28, 1960 or in the presence of various high boiling inert solvents. One of these, N-methylpyrrolidone, is described in U.S. Pat. No. 3,164,591 issued to Wilhelm E. Walles et al on Jan. 5, 1965. However, when attempts are made to apply the method taught by Walles et al, it is found that cyanuric acid is obtained in only about a 60 to 70 percent yield. Moreover, the crude acid obtained contains substantial percentages of the mono- and diamides of cyanuric acid, commonly referred to as ammelide and ammeline as well as other undesirable side reaction products and "color bodies". Thus, the crude end product of this procedure is frequently not sufficiently pure to be readily converted into a number of chlorinated secondary products of commercial interest. To make it so, it must first be subjected to further processing for such purification. This normally includes a digestion of the crude cynauric acid in a strong acid medium, e.g., 3 to 15 percent sulfuric or hydrochloric acid to selectively hydrolyze the acid soluble cyanuric acid amides to convert them back to cynauric acid. In general, an acid digestion is required whenever the concentration of ammeline or ammelide exceeds about 2 percent by weight of the cyanuric acid product.
Still a further problem with the method of Walles et al is that of producing a free flowing product suitable for further processing.
It has been found that when the reaction mass is allowed to fall below a temperature of about 100.degree. C., cyanuric acid and N-methylpyrrolidone combine to form an insoluble adduct which has a distinct tendency to "set up" into a hard concrete-like mass which must be broken up before any further use of the product can be made. Should this reaction occur during the processing steps used to remove the cyanuric acid from the reaction mass, such as filtering or centrifuging, drastic steps, sometimes involving disassembling the equipment and often damaging it, are necessary to remove the product and resume production.
It has also been found that even if the above adduct is broken up and then washed with water to remove residual solvent, it will often set up again into a concrete-like mass due to hydration of the cyanuric acid so that complete removal of the solvent to produce a purified product is difficult, if not impossible.
Lastly, the "color body" content is often sufficiently high that a supplemental bleaching operation is required to achieve a final product having a proper whiteness for commercial use.